1. Field of the Invention
The present invention relates to electronically controlled transmissions and more particularly to such transmissions having a plurality of solenoid valves grouped as a unit.
2. Description of the Prior Art
Automatic transmissions for use in vehicular applications are known in which multiple forward ratios from 3 to 5 speeds at the present time. Such systems can be mechanical and hydraulic controls that are load responsive to perform the drive and gear ratio selection. Such systems include one way clutches and various planetary gear sets along with clutches and brakes that are operated in response to the operation of grouped servo valves to execute shifts without interrupting the power flow from a power source to drive wheels of the vehicle.
In order to further improve transmission performance electronic transmission controls have been developed that combine electronic processing of vehicle operation to establish output signals to selectively energize the various solenoid valves to control the gear selection and for adapting the pressures within the transmission in accordance with the torque flow of the drive train. Examples of such transmissions are set-forth in U.S. Pat. No. 4,750,384.
It has been proposed that such electronically controlled transmissions have their solenoid valves operated so that the clutch apply and release sequences can be programmed in a manner to eliminate the need for one way clutch devices for smoothing gear shifts in the transmission.
Theoretically, such electronic controls appear to be able to produce shift smoothing without one-way clutches. In practice this has not been the experience of transmission designers.
The reason for a difference between theory and practice is in part due to the fact that the solenoid controlled valves are grouped in a valve housing on one part of the outer case of the transmission.
In such arrangements each of the solenoid valves that are grouped in one location on the transmission must be separately calibrated against a master. Likewise, the various clutch and brake units within the transmission must be separately calibrated. Once the clutches/brakes are assembled within a clutch housings it in turn is assembled within an outer transmission case that includes suitable internal flow patterns to connect the operating pistons of the clutch/brakes to one or more individual valves in a grouped valve arrangement thereon including the separately calibrated solenoid operated control valves.
Once assembled, variations in the separately calibrated components can produce unexpected lag between apply and release control steps at the clutch/brake components and as a consequence gear shift performance can be adversely affected.
It is known to provide a single solenoid controller for a clutch pack as set-forth in U.S. Pat. No. 4,750,384. However, in this arrangement the solenoid controller is mounted on the outer case of the transmission and the clutch pack is in a separate clutch housing. There is no suggestion that the solenoid controller be directly integrated within the clutch housing so that the clutch housing and solenoid controller can be calibrated as a single unit prior to assembly such that the operation thereof can be precisely determined prior to assembly.
Another problem with such arrangements is that the grouped solenoid operated control valves are located in a valve housing at a single location on the outer case of a transmission. This results in different length hydraulic flow paths to actuators located at radially and axially spaced locations with respect to a common centerline through the transmission. The pressure losses in such paths further add to the difficulty in properly calibrating the fluid and mechanical components of a transmission so as to be suitable for accurate electronic control without performance lag.
As a consequence the ability to affect various smoothing controls is limited not by the monitoring and electronic processing but rather by lags caused by differences in the imposition of pressure changes due to inadequate calibration.